1. Field of the Invention
The present invention relates generally to an airway adapter for use in connecting an endotracheal tube to a ventilation apparatus, and, in particular, to an airway adapter for use with a sidestream gas monitoring system that minimizes deadspace in the breathing circuit so that it is especially suited for use with a patient having a small endotracheal tube, i.e., an inner diameter of less than approximately 4.0 mm, and commensurately low tidal volumes.
2. Description of the Related Art
During medical treatment, a patient's exhalations are often times monitored and analyzed to determine the gaseous composition thereof. For instance, monitoring of the carbon dioxide (CO2) content of a patient's exhalations is often desirable. Typically, the CO2 (or other gaseous) content of a patient's exhalations is monitored by transferring a portion, or sample, of the patient's expired gas to a suitable sensing mechanism and monitoring system.
Monitoring of exhaled gases is typically accomplished utilizing either mainstream or sidestream monitoring system. In a mainstream monitoring system, the gaseous content of a patient's exhalation is measured in-situ in the patient's airway. In a sidestream monitoring system, on the other hand, a sample of the respired gases is removed from the patient's airway and transported through a gas sampling line to a sensing mechanism located some distance from the patient for monitoring.
In sidestream monitoring systems, it is desirable for a number of reasons to minimize the volume of gas removed, i.e., sample rate, from the airway for sampling. First, the lesser the volume of gas removed from the airway, the lesser the disturbance to the patient's ventilation. This is particularly important for patients having small tidal volumes, such as neonates and infants—which have average tidal volumes equivalent to about half the average adult tidal volume. Second, as the sample gas often contains contaminants and other constituents that must be removed prior to measurement, a reduced sample size, in turn, results in a smaller volume of contaminants and other constituents that must be removed from the sample in order to achieve accurate monitoring results. Third, when respiratory assistance is given in the operating room to patients under anesthesia, the sample gas will likely contain anesthetic agents that must be safely vented. Further, such anesthetic agents are often expensive and should not be wasted. Accordingly, the smaller the gas sample, the smaller the volume of anesthetic agents wasted and that must be properly handled.
Early gas sampling systems were limited by the response time of the sensing mechanism and required sampling rates of approximately 180–200 ml/min to achieve acceptable accuracy at higher respiratory rates. Such high sampling rates are not viable for low tidal volume patients, due to the unacceptable impact of the high sampling rate on the ventilation of the patient. Fortunately, recent advances in sensing technologies have permitted sampling rates to be reduced to approximately 50 ml/min, while still achieving acceptable accuracy. With viable low rate sensing mechanisms now available, design challenges for gas sampling systems for use with low tidal volume patients have shifted to providing a system in which a sample can be extracted from the breathing circuit without introducing excessive deadspace, i.e., void volume, into the breathing circuit, adding flow resistance to the breathing circuit, and/or losing the integrity of the sample at the sampling point in the breathing circuit.
There is often a discrepancy between the cross-sectional size of an endotracheal tube and the cross-sectional size of a ventilation tube. Thus, airway adapters are generally placed between the endotracheal tube and the ventilating tube to facilitate a relatively seamless connection therebetween. An airway adapter that is directly connected to an endotracheal tube is generally referred to as an endotracheal tube adapter.
If desired, an airway adapter may also include a sampling port through which gas samples are collected and transported to a sidestream gas monitoring system for analysis. Many conventional sampling airway adapters include a small opening extending through the wall of the adapter into the gas flow path through which gas samples are collected. However, termination of the sampling port in the wall of the adapter may permit contaminants and other constituents, which tend to collect along the inner wall of the adapter, to enter the sensing mechanism. Entry of such contaminants and other constituents into the sensing mechanism is undesired, as it may lead to inaccurate monitoring results. Accordingly, sampling airway adapters have been modified to include ports that extend beyond the inner surface of the wall of the adapter and into the center of the conduit through which gases flow.
Accurate gas analysis measurements depend, in part, upon the rapid and complete exchange of gases through the airway adapter, so as to maintain the characteristics or fidelity of the parameters being measured, e.g., the waveform of the gas to be analyzed. Internal mixing of respired gases and alterations in the waveform of the gas to be analyzed reduces the accuracy of the gas measurements and, thus, may produce results which do not accurately reflect the patient's medical status. In addition, it is desirable to prevent or minimize “unswept volume” in the airway adapter. As used herein, “unswept volume” refers to eddies or stagnant areas along the gas flow where the incoming gas fails to fully flush out the gas already in the airway adapter.
Airway adapters and the components to which they connect typically are manufactured as plastic injection moldings. To ensure suitably tight joints between airway adapters and the components which they connect, adapters and components are generally produced with slightly tapered portions so that one component fits tightly into the complementary component or adapter. However, there are fairly wide manufacturing tolerances for such plastic parts. As a result, loose-fitting connections will seal only when one component is pushed much farther into the other than is the case with tight-fitting connections. Consequently, the amount of deadspace produced by the connection of the components and/or airway adapters varies considerably and, for a tight-fitting pair of components, may be of considerable and undesirable magnitude, because, for example, the relatively large amount of deadspace may increase the effects of gas mixing and may result in rebreathing.
In view of this problem, airway adapters that seek to reduce the volume of deadspace introduced into the airway have been developed. Many of the proposed designs, however, are unsuitable for patients in which a very small flow is involved, e.g., neonatal patients, because even a small airway deadspace can cause significant mixing of the neonate's exhaled gases which, again, may produce inaccurate monitoring results.
One attempt to create an airway adapter suitable for neonatal patients that decreases the volume of deadspace in the airway, and purportedly maintains a smooth, laminar flow of gases, is described in PCT International Patent Application Publication WO 00/74756 to Oridion Medical, Ltd. (“hereinafter the '756 application”). FIG. 1 in the present application illustrates a first embodiment of an airway adapter 10 taught by the '756 application, and. FIG. 2 in the present invention illustrates a second embodiment of an airway adapter taught by the '756 application.
As shown in FIG. 1, adapter 10 includes a central passage 12 and a tubular insert 14 that is located inside the central passage. Insert 14 has an inside bore diameter 16 that substantially approximates the shape and size of the inside diameter of the tubular bore of the adapter 10. At the inner end of insert 14, the internal passageway opens out into a funnel shaped section 18, such that along the length of the funnel shaped section 18, internal diameter 16 of insert 14 gradually increases from the value it has along the majority of its length until it becomes equal to the internal diameter of central passage 12. A second end 20 of adapter 10 has a wide bore tubular opening 22 of a dimension suitable for connection to a standard ventilator tube. A sampling port arrangement 24 is built into the center section of airway adapter 10 to allow attachment of a gas sampling line thereto.
It can be appreciated that insert 14 can slide in a longitudinal direction, as indicated by arrows 28. However, the degree to which insert 14 extends into central passage 12 is limited by an outer end section 36 of passage 12 and a lip 34 provided on insert 14. The inward motion of the insert is arrested when lip 34 contacts end 36.
As shown in FIG. 2, the adapter includes a sleeve 38 instead of an insert, and utilizes a spring 40 to ensure positive contact between sleeve 38 and the adapter. Sleeve 38 slides on the outside of the wall 42, which defines the central passage through the airway adapter. The funnel shaped enlargement 44 in the airway bore is incorporated into the wall 42 of the central passage, and is thus fixed in this position. The sleeve seals against the inner wall 46 of the adapter where the internal diameter increases in a stepwise fashion. A seal 47 provided at one end of sleeve 38 is maintained in positive contact with an inner wall 48 of the adapter by spring 38.
While the two airway adapter embodiments discussed in the '756 application may provide a low volume airway adapter, they may not represent an optimal solution to the problems confronting this technology. For example, the sliding spring mechanism taught by the embodiments disclosed in the '756 application may become clogged with debris rendering it inoperative. Furthermore, the adapters taught by the '756 application require multiple parts that may be costly and time consuming to assemble, especially with respect to the sliding mechanism. Accordingly, other airway adapters that minimize deadspace, promote smooth flow of gases, minimize unswept volume and that are suitable for use with low tidal volume patients would be advantageous.